Failed NOx emission on 1990 80 series

[Danae Kranski]

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Please help,
I am a helicopter mechanic trying to get my Audi to pass Colorado’s
emissions test.  NOx = 6.8499 gpm 6. limit.  HC = 2.194gpm, 3. limit.
CO = 10.191gpm, 20.limit.  CO2 = 384.857gpm.  I have done the following:
Replaced dist. cap & rotor, Bosch spark plugs, thermostat-NAPA brand,
recovery tank pressure cap, air filter, engine oil and anti-freeze.  I
had an intermittent boil-over problem in the summer.  I found the
electric radiator fan intermittent and replaced it. I also tested the
thermo switch in the bottom of the radiator and the under hood after-run
switch and found both to be functioning. I disconnected most of the
electrical connectors and checked for corrosion but found none.  I
resorted to taking in the car for service.  The shop reviewed the
diagnostic trace reports and stated that only a high NOx reading was
uncommon to them.  They recommended and performed a BG induction
cleaning and I ran through a tank full of 44K treated fuel prior to the
retest.  The shop also jumpered to check the timing, cat had a delta of
100’F and checked the O2 sensor with a lab scope and found to be good.
They also mentioned that they enriched the mixture a little which
increased my HC from 1.889 to 2.194gpm.  I looked into the combustion
chamber with a Bend-A-Light and found a little carbon flake buildup on
the piston domes.  A compression check showed approx. 165psi per
cylinder which I thought was not bad for a 175K mile engine. Does anyone
have any suggestions to lower the NOx?
Regards
Richard
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